1. Field of the Invention
The present invention relates to an audio information processing apparatus, an audio information processing method, and a method of recording audio information on a recording medium, which are suitably arranged for a remaster apparatus or the like, for converting audio data recorded on a so-called compact disk (disc) (CD) into audio data for a video disk (disc) (digital video disk or digital versatile disk:DVD) which requires higher tone quality to remaster the audio data and, more particularly, to an audio information processing apparatus, an audio information processing method, and a method of recording audio information on a recording medium which generate harmonic sound on the basis of coded audio information and add the harmonic sound to original audio information as a predetermined band component to design extension or the like of the band of the original audio information.
2. Description of Related Art
It has been performed that an audio signal input as an analog signal is subjected to a sampling process at a predetermined sampling frequency and subjected to a quantization process to form coded audio data whose band is limited, and the audio data is recorded on a recording medium such as an optical disk. As a typical recording medium on which the audio data is recorded, a so-called compact disk (CD) is known. On the compact disk, 16-bit audio data which is subjected to a sampling process at a sampling frequency of 44.1 kHz to be formed is recorded.
A so-called digital video disk (DVD) obtained in the following manner is known. That is, image information including a lot of audio information and a lot of video information amount is highly efficiently compressed and coded by a moving picture compression processing apparatus such as an MPEG apparatus (MPEG : moving picture experts group), and the image information is recorded on an optical disk having a size (12-cm diameter) equal to that of a compact disk. The digital video disk has spread. On the digital video disk, 24-bit (or 20-bit) audio data formed by an analog audio signal is subjected to a sampling process at a sampling frequency of 96 kHz (or 88.2 kHz which is expected to be added as a new standard) is recorded.
For example, when an analog audio signal has a frequency band up to 48 kHz as indicated by a dotted line in FIG. 1, the analog audio signal is subjected to a sampling process at a sampling frequency of 44.1 kHz for a compact disk to be converted into 16-bit audio data. In this case, the audio data has a frequency characteristic in which a frequency band of 22.05 kHz or more is removed as indicated by an alternate long and short dash line in FIG. 1. In contrast to this, when an analog audio signal is subjected to a sampling process at a sampling frequency of 96 kHz for a digital video disk to be converted into 24-bit audio data, audio data having a frequency band up to 48 kHz can be formed like an analog audio signal as indicated by a solid line in FIG. 1.
Here, when an analog audio signal is converted into digital audio data, the resolution of the digital audio data depends on the number of quantization bits, and the frequency band of the digital audio data depends on a sampling frequency. For this reason, even if 16-bit data formed by performing a sampling process at a sampling frequency of 44.1 kHz for a compact disk is subjected to an over-sampling process at a sampling frequency of, e.g., 88.2 kHz, the 16-bit original audio data does not include sound having the frequency band of 22.05 kHz or more. For this reason, the frequency band of the audio data itself after the over-sampling process does not change.
Theoretically, the hearing sense of human being is limited to about 20 kHz. However, it is known that sound having a frequency band of 20 kHz or more and serving as the difference between the frequency band of a compact disk and the frequency band of a digital video disk as indicated by a diagonal line in FIG. 1 brings a rich hearing sense, though the sound cannot be heard.
For this reason, a technique in which the waveform of original audio information is shaped to emphasize or add harmonic and to record/reproduce rich sound is popularly studied. A technique for obtaining a non-linear waveform by using a conversion table is disclosed in Japanese Patent Application Laid-Open No. 5-127672; a technique for further performing a differential operation to form a complex non-linear waveform is disclosed in Japanese Patent Application Laid-Open No. 7-175478; a technique for performing a non-linear process after an over-sampling process to form harmonic is disclosed in Japanese Patent Application Laid-Open No. 7-66687; and a technique for performing a non-linear process after an over-sampling process to extract a wide-band component and adding the wide-band component to original audio information to form wide-band audio information is disclosed in Japanese Patent Application Laid-Open No. 7-236193.
However, since a conventional technique for emphasizing or adding harmonic is designed to use a conversion table for a non-linear process, a differential circuit, or a cube circuit, costs increase, and poor productivity is obtained because of a large circuit scale and a large chip size. In recent years in which price buster and downsizing are demanded, it is an important problem to provide a compact and high-performance audio information processing apparatus.